1. Field of the Invention
This invention is related to the domain of xcex3 radiation detection, the detector used being a semiconductor. It also relates to the field of spectrometric measurements in xcex3 imagery.
2. Discussion of the Background
Many types of detectors have been designed for detection of xcex3 radiation. The main innovation in xcex3 radiation detection techniques over the last 30 years has been the use of solid detectors based on semiconductors.
Detectors based on semiconductors convert xcex3 radiation in the material into energy directly without using any intermediate steps such as the emission of visible photons in the case of scintillators. This overcomes coupling problems that introduce loss of efficiency. The energy necessary to create an electron-hole pair in a semiconductor is much lower than in a gas or in a scintillator (about 4 eV for semiconductors compared with 30 eV in gases and 300 eV in photo-multiplier scintillator systems). Consequently, the number of free charges created for each detected photon is much higher, which can give better energy resolutions with low noise. Furthermore, the high atomic number and the high density of semiconductor materials make it possible to use detection volumes significantly smaller than the volumes of gas detectors or scintillators, while keeping the same quantic detection efficiency.
The use of these semiconductor materials as X or xcex3 radiation detectors implies the deposition of two electrical contacts on the surface of the material, at the terminals of which a polarization voltage is applied. Charge carriers, in other words electron-hole pairs created by the interaction of a xcex3 photon with the material, will separate under the action of the electrical field, the electrons migrating towards the positive electrode and the holes migrating towards the negative electrode. The capability of these charge carriers to migrate towards the electrodes without getting trapped by defects present in the semiconductor material will affect the energy resolution of the measured spectrum. This capability, also called the charge carrier transport property, is measured by the mobility and the life of the electrons and the holes.
The spectrometric measurement of incident photons consists of detecting the maximum number of photons within the detector volume, which requires a high thickness for better quantic detection efficiency, and precisely measuring the energy deposited by the photon, which requires an excellent efficiency for the collection of holes and electrons migrating towards the negative and positive electrodes respectively. These two parameters (quantic detection efficiency and charge carrier collection efficiency) are contradictory, since the former is proportional to the detector thickness, and the latter is inversely proportional to the detector thickness.
The quantic detection efficiency can be improved by optimizing the detector thickness compared with the field of application, since it only depends on the density and atomic number of the detector (for a given energy of incident photons).
However, the performances of current xcex3 detectors are limited by the presence of native defects in semiconductors which trap charge carriers during their migration towards electrodes, and correspondingly reduce their life and thus deteriorate the energy resolution of the detector. These native defects systematically appear during crystallogenesis of the semi-conducting material. There is a very abundant bibliography on the study of these defects that shows that the crystallogenesis of all high resistivity semiconductors that can operate at ambient temperature is not controlled sufficiently well to eliminate these defects.
The collection efficiency of charge carriers (holes and electrons) may be improved in different ways described in document EP-763 751.
This document also describes a process for the correction of the spectrometric measurement in the field of xcex3 photon detection.
This process consists of measuring the amplitude of the electronic contribution alone as a function only of its rise time (called the xe2x80x9celectronxe2x80x9d correction method) rather than measuring the total amplitude of the total integrated signal (electron+hole) as a function of its rise time (called the xe2x80x9cholexe2x80x9d correction method described in FR-2 738 693). The measurement, and consequently mathematical knowledge of the function correlating the amplitude of the electron signal and its rise time, is used to correct the measured amplitude associated with the interaction of each photon throughout the volume of the detector.
The advantage of the xe2x80x9celectronxe2x80x9d correction is that the existing relation between the amplitude of the electron signal and its rise time only depends on the mobility of the electrons that varies only slightly within an ingot and between two different ingots; the mobility depends mainly on the crystalline network. However, the xe2x80x9cholesxe2x80x9d correction depends on the life of the holes, and variations in this life can vary significantly within a single ingot. This life is imposed by the different defects created during crystallogenesis which is not well controlled. One consequence of these differences is that the xe2x80x9celectronxe2x80x9d correction can correct measurements in most semi-conducting materials, but this is not the case for the xe2x80x9cholexe2x80x9d correction.
The two documents mentioned above each describe a device for making or for correcting spectrometric measurements.
In both cases, a conventional preamplifier is used just at the detector output, followed by special electronics to measure the signal amplitude (electronic component in EP-763 751 and global signal in FR-2 738 693) and its rise time.
In both cases, the signal is measured using a conventional commercially available charge preamplifier well known to the expert in the subject (for example a 5093 preamplifier purchased from eV-Products).
FIG. 1 diagrammatically shows a detector 2, for example a CdTe detector and its preamplifier 4.
The charge deposited by the photon absorbed by detector 2 is integrated at the terminals of a capacitor Ci 8 called the xe2x80x9cintegrationxe2x80x9d capacitor. This capacitor counter-reacts with a resistance Ri 10 (through the use of an operational preamplifier) that xe2x80x9cdischargesxe2x80x9d the integrated charge at the capacitor terminals. The time constant Ri, Ci may be adapted as a function of the nature of the semiconductor and the charge to be measured.
In all cases, the darkness current associated with the detector is not measured and is therefore integrated at the terminals of the capacitor Ci, since its absolute value is still much too high. It is greater than the charge deposited by the photon interacting in the detector. It depends on the resistivity of the semiconductor and also on the polarization voltage applied to the detector terminals. However, the polarization voltage must be sufficient to collect charges deposited by each photon and thus measure their energy. The low charge transport properties (mobility and life) require polarization voltages (between 100 and 500 Volts depending on the detector thickness) for which the darkness current remains high. Consequently, a xe2x80x9cdecouplingxe2x80x9d capacitor 12 is inserted between the detector and the capacitor. This capacitor 12 eliminates the DC component of the darkness current and only transient pulses corresponding to the various interactions of photons in the detector are considered. This measurement principle is now the only possible configuration from the xe2x80x9celectronicxe2x80x9d point of view that can be used to measure the deposited charge without it being xe2x80x9cdrownedxe2x80x9d by the darkness current.
Unfortunately, this configuration is not electronically optimized in terms of the signal/noise ratio, since inserting the xe2x80x9cdecouplingxe2x80x9d capacitor 12 has the disadvantage of introducing a xe2x80x9cparasite capacitancexe2x80x9d between the input of the preamplifier and the electrical ground (voltage reference). The effect at the preamplifier output is to multiply the preamplifier noise voltage in the ratio xcexa3Cin/Ci, which is the sum of the capacitances with respect to the ground to Ci, the integration capacitance, which increases the noise component produced by the electronic amplification device.
The decoupling capacitor has been eliminated in a conventional spectrometry system (A. C. Huber et al. xe2x80x9cHigh Performance, thermoelectrically cooled X-Ray and Gamma Ray detectorsxe2x80x9d, Invited Paper at the International Conference on the Application of Accelerators in Research and Industry, Denton, Tex., USA, November 1994).
But the CdTe detector used then has to be cooled (to xe2x88x9230  C.) in order to reduce the average value of the darkness current. The performances are then good but Peltier elements have to be used for each detector. The result is that it is difficult to use, for example in an imagery system composed of 1600 detectors.
The invention is designed to solve this problem.
More particularly, the purpose of the invention is a semiconductor based device for the detection of gamma radiation comprising:
a semiconductor based detector with a resistivity exceeding 109 xcexa9xc2x7cm,
a charge preamplifier located directly at the detector output, without a decoupling capacitor and without a polarization resistance between the detector and the preamplifier,
a device for the use of a signal or a set of data representative of the variation with time of a signal output by the said semiconductor based detector in response to the interaction of a xcex3 photon with the semi-conducting material, this device comprising means of producing data or a signal representative of the rise time of the electronic component of the total signal output by the detector, in other words the component of the signal that corresponds to the collection of electrons originating from the interaction between each xcex3 photon and the semiconductor material.
The device for the use of a signal may also comprise:
means of producing data or a signal representative of either the total detected charge or the part of the total charge resulting from the collection of electrons,
or means of producing data or a signal representative of an amplitude of the electronic component of the signal, or an amplitude of the total signal.
Another purpose of the invention is a semiconductor based device for the detection of gamma radiation comprising:
a semiconductor based detector with a resistivity exceeding 109 xcexa9xc2x7cm,
an operational preamplifier located directly at the detector output, without a decoupling capacitor between the detector and the preamplifier,
a device for the use of a number of signals or a number of data sets, each being representative of the variation with time of a signal output by the semiconductor based detector, in response to the interaction of a xcex3 photon with the semi-conducting material, this device comprising:
means of producing data or a signal representative of the rise time of the electronic component of the total signal output by the detector, for each signal or data set, in other words the component of the signal that corresponds to the collection of electrons originating from the interaction of each xcex3 photon with the semi-conducting material,
and means of establishing a relation, or a correlation, between firstly a first data set representing the rise times of electronic components, and secondly a second data set representing total detected charges or charges resulting from the collection of electrons.
The device for the use of a number of signals may also include means of producing a signal or data representative of the maximum electrical charge or the maximum charge corresponding to at least part of the first and second data sets.
Another purpose of the invention is a semiconductor based device for the detection of xcex3 radiation comprising:
a semiconductor based detector with a resistivity exceeding 109 xcexa9xc2x7cm,
an operational preamplifier located directly at the detector output, without a decoupling capacitor between the detector and the preamplifier,
a device for the use of a signal, or a data set representative of the variation with time of a signal obtained by the semiconductor based detector in response to the interaction of a xcex3 photon to be measured with the semi-conducting material, comprising:
means of producing data or a signal representative of the rise time of the electronic component of the signal output by the detector, in other words the component of the signal that corresponds to the collection of electrons originating from the interaction of the xcex3 photon with the semi-conducting material,
means of producing data or a signal representative of either the total detected charge, or the part of the total charge resulting from the collection of electrons,
means of determining a maximum electrical charge, starting from:
signals or data about:
the rise time of the electronic component of the signal,
the total detected charge or the part of the total charge resulting from the collection of electrons,
and starting from a relation, or correlation, between firstly a first data set representative of the rise times of the electronic components, and secondly a second data set representing the total detected charges or charges resulting from the collection of electrons.
This device may also comprise means of correcting the measured charge starting from the maximum charge, the rise time corresponding to this maximum charge, and the actually measured rise time.
Advantageously, the charge preamplifier is fast and low noise.
A semiconductor based detector for the invention may be made of cadmium telluride (CdTe or CdTe:C1) or CdZnTe (for example CdZnTe:C1) or HgI2 or PbI2 or GaAl or PbIn; this material is derived from the xe2x80x9cHigh Pressure Bridgman methodxe2x80x9d, or the HPBM method, and the resistivity obtained is ten times greater than the resistivity of CdTe materials known in the past (obtained using the xe2x80x9cTraveling Heater Method (THM) or the xe2x80x9cBridgman Methodxe2x80x9d (BM) ), at ambient temperature. The low polarization current of the detector can then be carried by the counter-reaction resistance, by eliminating the connecting capacitor 12 and the polarization resistor 16. One advantage of this is the disappearance of the parasite capacitance of the connecting capacitor and therefore an improvement in the inherent noise of the electronics associated with the detector.
The invention has other advantages for detecting xcex3 photons. In particular, eliminating the decoupling capacitor and the associated polarization resistor reduces the number of components associated with each detector, which in the case of a matrix structure facilitates manufacture of the integrated electronics (in the case of an ASIC type production), and limits its cost. Furthermore, a low polarization voltage can be used at the detector terminals while maintaining excellent detection performances; this limits the darkness current and therefore aging of the detector, and the noise associated with the integrated charge measurement.
Use of the operation process correction method as described in document EP-763 751 (xe2x80x9celectronxe2x80x9d correction method) can very significantly improve the performances of a semiconductor based detector at ambient temperature, in terms of energy resolution and detection efficiency. The combination of this method with the use of a detector according to the invention with a resistivity greater than 109 xcexa9xc2x7cm, can maintain these excellent detector performances while using a very low polarization voltage at the terminals of the electrodes placed on each side of the detector faces. The very high resistivity of the detector used, and the use of the xe2x80x9celectronxe2x80x9d correction method increase the performances associated with the xe2x80x9celectronxe2x80x9d correction method by eliminating the decoupling capacitor at ambient temperature.
The combination of the high resistivity detector with the xe2x80x9celectronxe2x80x9d correction method considerably reduces the average value of the darkness current, and the associated noise. Obtaining a very low darkness current eliminates the decoupling capacitor and thus improves the noise associated with the measurement.
The invention also relates to a gamma-camera with a number N of small detectors operating at ambient temperature, each detector being of the type described above. Therefore, this gives a xcex3 imagery device with improved performances, particularly in terms of energy resolution and image contrast. Furthermore, manufacturing costs are reduced by the elimination of two electronic components, the decoupling capacitor and the polarization resistor in each detector.
Another purpose of the invention is a process for the detection of xcex3 radiation embodying the device described above.